The present invention relates to a method of producing an acoustic diaphragm constituted by a core member made of a web material such as metal foil and surface materials adhered to both surfaces of the core member, as well as to a method of producing the same. More particularly, the invention aims at providing an acoustic diaphragm having an enlarged reproducible frequency range and a flat frequency characteristic to improve the efficiency of the speaker, as well as to a method of producing such an acoustic diaphragm.
Conventional acoustic diaphragms for speakers have a core member of a honey-comb structure consisting of a multiplicity of laminated metal foils, the adjacent metal foils being adhered at a regular pitch. In this conventional acoustic diaphragm, because of a directionality of rigidity of the honey-comb core member, the nodal line generated during resonance of the diaphragm takes an oval shape. As a result, the nodal points in the direction of the major axis of the oval shape are positioned outside the voice coil of the acoustic diaphragm, whereas the nodal points in the direction of the minor axis of the oval shape are positioned inside of the voice coil.
For this reason, in the conventional acoustic diaphragm, a peak of sound pressure is generated at the resonance frequency of the acoustic diaphragm and the range of flat sound pressure level is inconveniently narrowed.
In order to overcome this problem it has been proposed to increase the resonance frequency so as to enlarge the flat sound pressure range, by adopting a voice coil bobbin having an oval cross-section. When the coil bobbin having an oval cross-section is used, the voice coil also has an oval shape which in turn necessitates a magnetic circuit having an oval magnetic gap. In consequence, the production process is impractically complicated and the cost of production of such a speaker is raised uneconomically.